Concrete blocks, also referred to as concrete masonry units (CMUs), are typically manufactured by forming them into various shapes using a concrete block machine employing a mold frame assembled so as to form a mold box. A mold cavity having a negative of a desired shape of the block to be formed is provided within the mold box. A support board, or pallet, is moved via a conveyor system onto a pallet table. The pallet table is moved upward until the pallet contacts and forms a bottom of the mold box. The cavity is then filled with concrete by a moveable feedbox drawer.
As soon as the mold is filled with concrete, the feedbox drawer is moved back to a storage position and a plunger, or head shoe assembly, descends to form a top of the mold. The head shoe assembly is typically matched to the top outside surface of the mold cavity and is hydraulically or mechanically pressed down on the concrete. The head shoe assembly compresses the concrete to a desired pounds-per-square-inch (psi) rating and block dimension while simultaneously vibrating the mold along with the vibrating table, resulting in substantial compression and optimal distribution of the concrete throughout the mold cavity.
Because of the compression, the concrete reaches a level of hardness that permits immediate stripping of the finished block from the mold. To remove the finished block from the mold, the mold remains stationary while the shoe and pallet table, along with the corresponding pallet, are moved downward and force the block from the mold onto the pallet. As soon as the bottom edge of the head shoe assembly clears the bottom edge of the mold, the conveyor system moves the pallet with the finished block forward, and another pallet takes its place under the mold. The pallet table then raises the next pallet to form a bottom of the mold box for the next block, and the process is repeated.
For many types of CMU's (e.g., pavers, patio blocks, light weight blocks, cinder blocks, etc.), but for retaining wall blocks and architectural units in particular, it is desirable for at least one surface of the block to have a desired texture, such as a stone-like texture. One technique for creating a desired texture on the block surface is to provide a negative of a desired pattern or texture on the side walls of the mold. However, because of the way finished blocks are vertically ejected from the mold, any such pattern or texture would be stripped from the side walls unless they are moved away from the mold interior prior to the block being ejected.
One technique employed for moving the sidewalls of a mold involves the use of a cam mechanism to move the sidewalls of the mold inward and an opposing spring to push the sidewalls outward from the center of the mold. However, this technique applies an “active” force to the sidewall only when the sidewall is being moved inward and relies on the energy stored in the spring to move the sidewall outward. The energy stored in the spring may potentially be insufficient to retract the sidewall if the sidewall sticks to the concrete. Additionally, the cam mechanism can potentially be difficult to utilize within the limited confines of a concrete block machine.
A second technique involves using a piston to extend and retract the sidewall. However, a shaft of the piston shaft is coupled directly to the moveable sidewall and moves in-line with the direction of movement of the moveable sidewall. Thus, during compression of the concrete by the head shoe assembly, an enormous amount of pressure is exerted directly on the piston via the piston shaft. Consequently, a piston having a high psi rating is required to hold the sidewall in place during compression and vibration of the concrete. Additionally, the direct pressure on the piston shaft can potentially cause increased wear and shorten the expected life of the piston.